The invention relates to a backlight unit and a liquid crystal display, and in particular to a backlight unit with a film positioning structure.
Liquid crystal displays are used in a variety of electronic devices, including personal digital assistant (PDA), notebook and desktop computer monitors, mobile phones, and TVs. Some liquid crystal displays are rotatable between different angles, such as, from 0° to 90°, 180°, or 270°. Larger panels, however, causes weight to increase accordingly, incurring a load problem during rotation.
FIG. 1A is a schematic view of a conventional backlight unit 10A for a liquid crystal display, disclosed in Taiwan patent No. 449048. In FIG. 1A, the backlight unit 10A comprises at least one light source (not shown), a plurality of optical films 120, a frame 15, and a fixing structure for the optical films 120. The light source is disposed between the optical films 120 and the frame 15. Light emitted by the light source passes through the optical films 120 such as a diffusion sheet, a prim sheet, and the like, thereby uniformly distributing the light. Note that FIG. 1A eliminates the upper and lower housings and the panel to clearly show the backlight unit 10A.
Extended portions 121, 122, 123, and 124 extend from a periphery of each optical film 120. Each extended portion 121, 122, 123, and 124 comprises holes 121a, 122a, 123a, 124a in the centers thereof. The frame 15 comprises four grooves 151, 152, 153, and 154, respectively corresponding to the extended portions 121, 122, 123, and 124. Protrusions 151a and 152a are respectively formed in the grooves 151 and 152, and hooks 153a and 154a are respectively formed in the grooves 153 and 154. When fixing the optical films 120 to the frame 15, the protrusions 151a and 152a and the hooks 153a and 154a pass through the holes 121a, 122a, 123a, 124a of the extended portions 121, 122, 123, and 124 such that the extended portions 121, 122, 123, and 124 are respectively disposed in the grooves 151, 152, 153, and 154. Thus, the optical films 120 are securely fixed in the frame 15.
Although the films 120 are securely fixed in the frame 15, manufacturing processes, such as, modeling of the grooves and fixing elements such as hooks and protrusions is complicated. The capital cost is thus increased. Furthermore, if temperature varies, the optical films 120 may expand or contract. As a result, stress is concentrated at the four fixing points at corners of the films 120; hence, Mura defects occur accordingly.
Taiwan patent No. 552440 attempts to solve the above mentioned problem. The backlight module 10B has an improved fixing structure for the optical films to prevent Mura defects. As shown in FIGS. 1B and 1C, the optical films 120 are disposed above the frame 15b. Only one edge of each optical film 120 comprises an extended portion 125 with a hole 125b formed thereon. Before assembling the optical films 120, a jig or other tool is required to fix each optical film 120 to align the extended portion 125 to a groove 150 of the frame 15b. After the optical films 120 are installed on the frame 15b, another fixing element such as a pin 120a and fixing base 120b are required to secure the optical films 150 thereon. Although the optical films 120 are securely fixed, additional time is required for the installing and detaching process. Moreover, alignment is difficult and requires additional alignment tools and jigs.
Furthermore, when positioning at different angles, for example, rotating the liquid crystal display to 180°, all the weight of the optical films 120 are concentrated at the single fixing point P. Since the fixing position is asymmetrical, during expansion and contraction of the films, stress is concentrated on the fixing point P, thus causing deformation of the optical films 120. This also results in Mura defects and uneven optical efficiency.